The present disclosure relates to flight control systems, and more particularly, to aircraft fly-by-wire control systems.
Many vehicles, including helicopters for example, use fly-by-wire (FBW) systems to control flight operation. With these systems, it is possible for the flight control system of a FBW aircraft to provide a variety of tactile cues (such as power cues, level flight cues, control limit cues, etc.) to the pilot. For example, inverse plant with airspeed gain scheduling can be utilized to optimize control laws for different flight regimes such as, for example, low airspeed regimes and high airspeed regimes. In order to maintain aircraft stability, various FBW systems use real-time, i.e., real, airspeed to determine when to change FBW system control laws and gain parameters between high speed and low speed regimes.
Conventional FBW systems establish a default fixed gain value for controlling flight in order to ensure safe transition from high speed to low speed regimes if loss of all measured real airspeed occurs. Another method utilizes global position satellite (GPS) data to estimate airspeed when loss of measured real airspeed occurs. For example, inertial data from an inertial navigation system (INS) is used to update global position satellite (GPS) data for determining position, velocity, and attitude. The position, velocity and attitude determined from the GPS data are in turn utilized to estimate a true airspeed based on the geometric relationship between airspeed, wind speed, and ground speed. Some FBW systems, however, do not provide a definitive default fixed gain value that can be used for high speed to low speed transition. In addition, not all FBW aircraft systems have access to INS or GPS data necessary to estimate airspeed.